Unbreak accidental back-compat break with event optimization

This was supposed to be purely additive. See here for more details:

#1949 (comment)

docs/change-log.md

@@ -31,7 +31,7 @@
 #### News
 
 - API: Introduction of `m.redraw.sync()` ([#1592](https://github.com/MithrilJS/mithril.js/pull/1592))
-- API: Event handlers may also be objects with `handleEvent` methods ([#1939](https://github.com/MithrilJS/mithril.js/issues/1939)).
+- API: Event handlers may also be objects with `handleEvent` methods ([#1949](https://github.com/MithrilJS/mithril.js/pull/1949), [#2222](https://github.com/MithrilJS/mithril.js/pull/2222)).
 - API: `m.route.link` accepts an optional `options` object ([#1930](https://github.com/MithrilJS/mithril.js/pull/1930))
 - API: `m.request` better error message on JSON parse error - ([#2195](https://github.com/MithrilJS/mithril.js/pull/2195), [@codeclown](https://github.com/codeclown))
 - API: `m.request` supports `timeout` as attr - ([#1966](https://github.com/MithrilJS/mithril.js/pull/1966))

render/render.js

@@ -802,13 +802,20 @@ module.exports = function($window) {
 	// 4. The event name is remapped to the handler before calling it.
 	// 5. In function-based event handlers, `ev.target === this`. We replicate
 	//    that below.
+	// 6. In function-based event handlers, `return false` prevents the default
+	//    action and stops event propagation. We replicate that below.
 	function EventDict() {}
 	EventDict.prototype = Object.create(null)
 	EventDict.prototype.handleEvent = function (ev) {
 		var handler = this["on" + ev.type]
-		if (typeof handler === "function") handler.call(ev.target, ev)
+		var result
+		if (typeof handler === "function") result = handler.call(ev.target, ev)
 		else if (typeof handler.handleEvent === "function") handler.handleEvent(ev)
 		if (typeof onevent === "function") onevent.call(ev.target, ev)
+		if (result === false) {
+			ev.preventDefault()
+			ev.stopPropagation()
+		}
 	}
 
 	//event

render/tests/test-event.js

@@ -32,6 +32,29 @@ o.spec("event", function() {
 		o(onevent.this).equals(div.dom)
 		o(onevent.args[0].type).equals("click")
 		o(onevent.args[0].target).equals(div.dom)
+		o(e.defaultPrevented).equals(false)
+		o(e.$propagationStopped).equals(false)
+	})
+
+	o("handles onclick returning false", function() {
+		var spy = o.spy(function () { return false })
+		var div = {tag: "div", attrs: {onclick: spy}}
+		var e = $window.document.createEvent("MouseEvents")
+		e.initEvent("click", true, true)
+
+		render(root, [div])
+		div.dom.dispatchEvent(e)
+
+		o(spy.callCount).equals(1)
+		o(spy.this).equals(div.dom)
+		o(spy.args[0].type).equals("click")
+		o(spy.args[0].target).equals(div.dom)
+		o(onevent.callCount).equals(1)
+		o(onevent.this).equals(div.dom)
+		o(onevent.args[0].type).equals("click")
+		o(onevent.args[0].target).equals(div.dom)
+		o(e.defaultPrevented).equals(true)
+		o(e.$propagationStopped).equals(true)
 	})
 
 	o("handles click EventListener object", function() {
@@ -52,6 +75,30 @@ o.spec("event", function() {
 		o(onevent.this).equals(div.dom)
 		o(onevent.args[0].type).equals("click")
 		o(onevent.args[0].target).equals(div.dom)
+		o(e.defaultPrevented).equals(false)
+		o(e.$propagationStopped).equals(false)
+	})
+
+	o("handles click EventListener object returning false", function() {
+		var spy = o.spy(function () { return false })
+		var listener = {handleEvent: spy}
+		var div = {tag: "div", attrs: {onclick: listener}}
+		var e = $window.document.createEvent("MouseEvents")
+		e.initEvent("click", true, true)
+
+		render(root, [div])
+		div.dom.dispatchEvent(e)
+
+		o(spy.callCount).equals(1)
+		o(spy.this).equals(listener)
+		o(spy.args[0].type).equals("click")
+		o(spy.args[0].target).equals(div.dom)
+		o(onevent.callCount).equals(1)
+		o(onevent.this).equals(div.dom)
+		o(onevent.args[0].type).equals("click")
+		o(onevent.args[0].target).equals(div.dom)
+		o(e.defaultPrevented).equals(false)
+		o(e.$propagationStopped).equals(false)
 	})
 
 	o("removes event", function() {

test-utils/domMock.js

@@ -364,10 +364,18 @@ module.exports = function(options) {
 						e.preventDefault = function() {
 							prevented = true
 						}
+						Object.defineProperty(e, "defaultPrevented", {
+							enumerable: true,
+							get: function () { return prevented }
+						})
 						var stopped = false
 						e.stopPropagation = function() {
 							stopped = true
 						}
+						Object.defineProperty(e, "$propagationStopped", {
+							enumerable: true,
+							get: function () { return prevented }
+						})
 						e.eventPhase = 1
 						try {
 							for (var i = parents.length - 1; 0 <= i; i--) {